Opto-electronic scale-reading apparatus

ABSTRACT

An opto-electronic scale-reading apparatus comprising a read head and a scale supported for displacement relative to each other for the incremental reading of marks provided on the scale. The read head includes a light source for producing light rays incident on the scale, an index grating for diffracting readable rays into fringes in at least one order of diffracton, and an analyzer grating for converting the fringes into light modulations at a rate which is a function of the rate of displacement between the read head and the scale. The scale comprises an elongate member having marks spaced in the direction of its length and spaces defined between the marks. The marks and spaces are defined by respective first and second surfaces of surface portions of the members, the first surfaces laying at an angle to adjacent second surfaces so that incident rays are reflected by the first surfaces in a first direction to be readable by the read head in the first direction. The incident rays are reflected by the second surfaces in a second direction such that rays reflected in the second direction are substantially not readable in the first direction.

BACKGROUND OF THE INVENTION

This invention relates to opto-electronic scale-reading apparatus of thekind wherein a read head and a scale are supported for movement onerelative to the other for the incremental reading of marks provided onthe scale, the scale is illuminated by incident light, the read head isarranged to view the scale in a given direction, and the scale comprisesan elongate member having said marks spaced in the direction of thelength thereof, spaces being defined between the marks.

GB-932,481 discusses the difficulty of providing good contrast betweenthe marks and spaces at a metallic surface and describes a scale whereinsuch contrast is sought to be achieved by marks having highly reflectivesurfaces and spaces having diffusively reflective surfaces. However, themarks are applied to a flat surface of the scale member by aphoto-resist process so that both the marks and the spaces have parallelsurfaces and act as if they are substantially co-planar. This can stilllead to difficulties in providing good contrast especially if the scalesurface is contaminated.

GB-1,516,536 discusses the difficulty of contamination of the scale anddescribes apparatus wherein both the marks and the spaces areilluminated by diffuse light. However, such diffuse illuminationnecessarily reduces contrast, and contamination can reduce the contrastbetween the marks and the spaces still further because the marks andspaces would be at parallel substantially co-planar surfaces.

EP-0160811 describes a diffraction grating having regions of differentperiodicities with a view to forming diffraction fringes incorrespondingly different positions. The different fringes define codedreference marks in an incremental opto-electronic scale-readingapparatus. The grating is a phase grating formed by a square or by asawtooth profile illuminated by coherent light but the grating linesextend in the direction of the length of the incremental scale and donot themselves form the basis for an incremental count of position.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide in anopto-electronic scale-reading apparatus a scale having improved opticalcontrast between the marks and the spaces especially where the marks andspaces are defined at a metallic surface.

More specifically, an opto-electronic scale-reading apparatus isprovided which comprises a read head and a scale supported fordisplacement relative to each other for the incremental reading of marksprovided on the scale. The read head includes a light source forproducing light rays incident on the scale, an index grating fordiffracting readable rays into fringes in at least one order ofdiffraction, and an analyzer grating for converting the fringes intolight modulations at a rate which is a function of the rate ofdisplacement between the read head and the scale.

The scale comprises an elongate member having marks spaced in thedirection of its length and spaces defined between the marks. The marksand spaces are defined by respective first and second surfaces ofsurface portions of the members, the first surfaces lying at an angle toadjacent second surfaces so that incident rays are reflected by thefirst surfaces in a first direction to be readable by the read head inthe first direction. The incident rays are reflected by the secondsurfaces in a second direction such that rays reflected in the seconddirection are substantially not readable in the first direction.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of this invention will now be described with reference tothe accompanying drawings wherein:

FIG. 1 is a sectional elevation of a first example of a scale-readingapparatus and shows a first example of a scale.

FIG. 2 is a view on the line II--II in FIG. 1.

FIG. 3 is a sectional elevation of a second example of a scale-readingapparatus also shows a second example of a scale.

FIG. 4 is an enlarged detail of FIG. 3.

FIG. 5 shows the profile of a third example of a scale.

FIG. 6 shows the profile of a fourth example of a scale.

FIG. 7 shows the profile of a fifth example of a scale.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, the scale, 10, comprises an elongate scale member 11 havingmarks 12 and spaces 13 provided thereon alternately along the length ofthe member 11. A read head 20 includes a point source 21 for incidentlight rays 24. The source 21 is positioned to illuminate the scale 10through the intermediary of a collimating lens 22 and a transmissivegrating 23. The read head 20 and the scale 10 are supported for relativemovement in the direction of the length of the scale, i.e. the directionof the spacing of the marks 12, in a manner known per se. The grating 23divides the incident light into individual light sources 25. The scale10 constitutes a reflective diffraction grating which diffracts thelight from the sources 25 in at least one order of diffraction and so asto produce diffraction fringes 26. The spacing between the scale 10 andthe grating 23 is such that the fringes 26 occur in the plane 23A of thegrating 23. The grating 23 cooperates to generate, at its side remotefrom the scale, light modulations 27 at the same rate as that at whichthe marks 12 pass the read head 20 during its movement relative to thescale 10. The lens 22 focusses the reflected and modulated light, 28,onto an opto-electronic transducer 29 which generates correspondingelectric signals 30. An incremental or decremental count of the signals30 at a counter 31 provides a measure of the relative position of theread head 20 and the scale 10 during their relative movement.

The apparatus is further described with reference to the orthogonaldirections X, Y, Z. The scale 10 is elongate in the X-direction, themarks 12 and the lines, 23B, of the grating 23 are elongate in theY-direction (FIG. 2). The scale 10 has a reference plane 10A includingthe X and Y-directions. The read head 20 has an optical axis 20A lyinggenerally in a plane including the X and Z-directions but may, as shown,extend only in the Z-direction. The axis 20A defines the readingdirection of the read head 20.

The marks 12 are defined by first surfaces 40 which lie parallel to theplane 10A of the scale 10, i.e. the axis 20A is normal to the surfaces40. The surfaces 40 are provided at portions 41 of the member 11. Thespaces 13 are defined by surfaces 50 formed by portions 51 of the member11. The portions 51 have a sawtooth profile extending out of the plane10A. The surfaces 50 are inclined relative to the surfaces 40 andrelative to the plane 10A at an angle E of e.g. 30°. It will be clearthat incident light from the sources 25 falling onto the surfaces 40 issubstantially reflected by the surfaces 40 back to the grating 23 asshown by rays 40A,40B. Thus the surfaces 40 can participate in thediffraction mechanism. Distinct from that, an incident ray 50A fallingonto the surfaces 50 is reflected, by virtue of the inclination of thesesurfaces, in a direction away from the read head 20 as shown by a ray50B, and diffraction at these surfaces would not produce orders readableby the head 20. If any of the rays 40B are reflected back from thegrating 23 onto the surfaces 50, they will be deflected by the surfaces50 at least substantially away from the grating 23 and will notsubstantially diminish the contrast between the marks 12 and the spaces13 as far as the read head 20 is concerned. The surfaces 40,50 arepreferably specular.

A typical pitch P for the scale described with reference to FIG. 1 is a0.020 mm, use being make of infrared for the incident light. Since thescale has to be of diffraction quality the tolerance for the position ofthe marks has to be accordingly fine. See "Diffraction Gratings" by M CHutley published 1982 by Academic Press, London, page 76.

In FIG. 3, the apparatus is similar to that shown in FIG. 1 (like partsbeing given like reference numerals in both FIGURES) except as follows.The read head 20 is provided with two gratings 32,33. A light source 34,of infrared or white light, is positioned to direct divergent incidentrays 24 onto the scale 10 from a position outside the optical paththrough the gratings 32,33 and obliquely to the plane 10A. In thisexample, it is the scale 10 which produces the individual light sources25. The grating 32 generates the fringes 26, the grating 33 generatesthe light modulations 27, and the lens 22 collects the modulated light28 for focussing onto the transducer 29. Such an arrangement ofgratings, constitutes a spatial filter as described in WO 86/03833. Thatis, the read head is relatively unaffected if the marks 12 are not ofdiffraction quality and a wider tolerance can be accepted regardingposition, periodicity or surface finish of the marks compared to thetolerance mentioned in connection with FIG. 1. Also, the filter has auseful tolerance to the marks by being diffusely reflective as byvariation in the surface shape or surface finish of the marks. This canbe of advantage because such diffuse reflectivity allows a relaxation oftolerances in the relative position of the read head and the scale aboutaxes lying in the Y or in the X-direction.

Referring now also to FIG. 4, the marks 12 are defined by first surfaces42 which lie at an angle A (FIG. 4) to the reference plane 10A and at acomplementary angle B to the axis 20A. The spaces 13 are defined bysurfaces 52 which lie at an angle C to the plane 10A and at acomplementary angle D to the axis 20A. Thus the surfaces 42,52 form twosides, at an angle E, of a triangle, the third side of which isnotionally formed by the plane 10A. In other words the surfaces 42,52are jointly formed by a portion 43 of the member 11 having a triangularprofile extending out of the plane 10A. By virtue of the angles A, B thesurfaces 42 reflect the incident light 24 toward the read head 20. Inother words, the angles A, B are such that the light is readable by thegrating 32 by reflection at the surfaces 40, and the angles C, D aresuch that the incident light 24 is deflected away from the read head andis therefore not readable by the grating 32.

In view of the tolerances mentioned there may be irregularities in thepitch P of the surfaces 42 or in the height of the surfaces 42 above theplane 10A. Also the surfaces 42 may have a surface finish renderingreflection from them non-specular although a specular finish may bedesirable for optical efficiency. However, the surfaces 52 may be ofspecular finish to avoid stray reflections in the direction of the readhead, i.e. in the Z direction. Regarding back reflection from thegrating 32 it will be seen that such reflection is diverted both by thesurfaces 42 and 52 away from the read head 20. The angles A, C may eachbe 30°.

FIG. 5 shows a scale 10 suitable for use with the read head 20 shown inFIG. 1 and having second surfaces 54 formed by a triangular projection55 from the plane 10A i.e. the plane of the first surfaces 40, thesecond surface 54 being formed by the two sides or facets 551,552 of theprojection 55. In a modification shown in broken lines, the secondsurfaces are formed by a recess 553 out of the plane 10A.

FIG. 6 shows a scale 10 suitable for the read head 20 shown in FIG. 3and having first surfaces 46 and second surfaces 56 wherein each pair ofthe first and second surfaces 46,56 is formed by respective sides of acommon projection 57 of the member 11 out of the reference plane 10A. Asshown, the first surfaces reflect incident rays 24 as rays 46Asubstantially toward the read head i.e. in the direction of the axis 20Awhile the second surfaces 56 reflect the incident rays as rays 56A awayfrom the axis 20A.

FIG. 7 shows a scale 10 suitable for oblique illumination as shown inFIG. 3 and having first and second surfaces 48,58 formed by commonportions 59 of the member 11. The surfaces 48 are inclined to thereference plane 10A to reflect incident rays 24 as rays 48A toward theread head. The second surfaces 58 are formed by facets 581,582 whichdefine a recess out of the direction of the incident rays 24 so that thefacets 581,582 are in shadow, denoted S, and no light is receivabletherefrom by the read head 20. The facets 582 are at least partiallycurved as shown. The facets 581 lie in the plane 10A and insofar as theydo not lie in the shadow they reflect the oblique rays 24 away from theread head as shown at 581A. The profile shown in FIG. 7 is a compositeof planar and rounded surfaces and shows the flexibility available inthe design of a particular profile. The angle A between the surfaces 48and the reference plane may have a tolerance of ±0.5°.

Any one of the scales described may be provided with a transparentplastics coating, e.g. an acrylic plastics, 60 as shown in FIG. 7 toprotect the profile of the first and second surfaces. The coating has aflat outer surface 61. When designing the profile account has to betaken of refraction at the surface 61. Inasmuch as it is desired toavoid ordered diffraction to benefit from relaxation of tolerance asmentioned in connection with FIG. 3, a required degree of diffusion canbe introduced by the appropriate choice of the plastics material.

Generally, it will be seen that in any of the scale profiles describedthe angular relationship between the first and second surfaces operatesto provide optical distinction, i.e. contrast, between the marks andspaces. Further, the first and second surfaces can be formed in the samematerial, i.e. there is no need to apply separate material to asubstrate to establish the marks. The pitch of the marks may be in theorder of 20 micron and a typical angle between the plane 10A and thefirst or second surfaces is in the range 25° to 35°, but basically, theangle E between the first and second surface need only be large enoughto ensure that the read head does not read reflection or diffractionfrom the second surface.

I claim:
 1. Opto-electronic scale-reading apparatus comprising a readhead and a scale supported for displacement one relative to the otherfor the incremental reading of marks provided on the scale, the readhead including a light source adapted to produce light rays incident onthe scale, the read head being arranged to read the scale in a firstdirection; the scale comprising an elongate member having marks spacedin the direction of the length thereof and spaces defined between themarks, the marks and spaces being defined by respective first and secondsurfaces of surface portions of the member, wherein any first surfacelies at an angle to an adjacent said second surface so that, inoperation, the incident rays are reflected by the first surfacessubstantially in said first direction to be readable by said read head,and the incident rays are reflected by the second surfaces in a seconddirection such that rays reflected in the second direction aresubstantially not readable in the first direction; andwherein said readhead comprises an index grating for diffracting the readable rays intofringes in at least one order of diffraction, and an analyzer gratingfor converting the fringes into light modulations at a rate which is afunction of the rate of displacement between the read head and thescale.
 2. Opto-electronic scale-reading apparatus comprising a read headand a scale supported for movement one relative to the other for theincremental reading of marks providing on the scale, the read headincluding a light source adapted to produce light rays incident on thescale, the read head being arranged to read the scale in a givendirection;the scale comprising an elongate member having marks spaced inthe direction of the length thereof and spaces defined between themarks, wherein the marks and spaces are defined by respective first andsecond surfaces of surface portions of the member, wherein any firstsurface lies at an angle to an adjacent said second surface so that, inoperation, the incident rays are reflected by the first surfaces so asto be readable in said given direction, and the second surfaces are atleast partially shadowed by said surface portions so that the secondsurfaces are substantially not readable in said given direction;andwherein said read head comprises an index grating for diffracting thereadable rays into fringes in at least one order of diffraction, and ananalyzer grating for converting the fringes into light modulations at arate which is a function of the rate of displacement between the readhead and the scale.
 3. Opto-electronic scale-reading apparatuscomprising a member provided with a first periodic structure defining ascale, a read head provided with a second and a third periodicstructure, and a light source for illuminating the scale;the scale beingadapted for directing light incident thereon toward the read head, thefirst and second periodic structures cooperating optically to generate afringe pattern at the third periodic structure, the scale and the readhead being supportable for relative movement in the direction of theperiodicity of the first periodic structure and consequential relativemovement between the fringe pattern and the third periodic structureenabling determination of the extent of relative movement between thescale and the read head; wherein the first periodic structure is definedby alternate first and second surfaces provided on said member and ofwhich any first surface lies at an angle to an adjacent said secondsurface so that the light incident on the scale is directed toward theread head only by said first surfaces.
 4. Apparatus according to claim3, wherein the first surfaces lie substantially in a common plane, andthe second surfaces are inclined to the first surfaces.
 5. Apparatusaccording to claim 4, wherein the second surfaces are formed by surfaceportions, said surface portions being projections or recesses of themember relative to said common plane.
 6. Apparatus according to claim 3,wherein the first surfaces are inclined to a reference plane and thesecond surfaces are inclined to the reference plane in a sense oppositeto that in which the first surfaces are inclined.
 7. Apparatus accordingto claim 3, wherein at least one of the first and second surfaces iscurved.
 8. Apparatus according to claim 3, wherein each pair of thefirst and second surfaces is defined by respective sides of a commonprojection or recess of the member relative to a reference plane. 9.Apparatus according to claim 3 wherein said second surfaces arepositioned to direct the incident light away from the read head. 10.Apparatus according to claim 3 wherein said member has surface portionson each of which a said first and a said second surface is defined, andsaid light source and said surface portions are so arranged that saidsurfaces lie at least partially in shadows cast by said surfaceportions.
 11. Apparatus according to claim 3, wherein said second andthird periodic structures are arranged for light from said light sourceto interact in succession with said second, first and third periodicstructures.
 12. Apparatus according to claim 11, wherein said second andthird periodic structures are defined by a single grating situatedbetween said light source and said scale.
 13. Apparatus according toclaim 11, wherein said first periodic structure has a two-dimensionalextent in a reference plane, said second periodic structure is arrangedfor light interacting therewith to pass therefrom toward the firstperiodic structure in a mean direction normal to said reference plane,and said first surfaces of the scale are reflective surfaces lying in aplane parallel to said reference plane.
 14. Apparatus according to claim3, wherein said second and third periodic structures are arranged forlight from said source to interact in succession with said first, secondand third periodic structures.
 15. Apparatus according to claim 14,wherein said first periodic structure has a two-dimensional extent in areference plane, said light source is arranged for light therefrom to beincident on the first periodic structure in a mean direction at an angleoblique to said reference plane, said first surfaces lying at an angleto said reference plane such as to reflect the light toward the readhead.
 16. Apparatus according to claim 15 wherein said second surfaceslie at an angle to said reference plane such as to reflect the lightaway from the read head.
 17. Apparatus according to claim 14, whereinsaid first periodic structure is not of diffraction quality. 18.Apparatus according to claim 14 wherein said first periodic structure isadapted for light interacting with at least said first surfaces to bediffused by such interaction.
 19. Apparatus according to claim 3, saidrelative movement between the fringe pattern and the third periodicstructure causing the fringes of said fringe pattern to generate lightintensity modulations at the side of the third periodic structure remotefrom the first and second periodic structures, and the apparatuscomprising an opto-electronic transducer and a focussing means forfocussing the modulations pertaining to the respective fringes of saidpattern on to said transducer thereby to produce a signal representingthe sum of the intensities of the individual modulations.
 20. Apparatusaccording to claim 3, comprising a layer of a substantially transparentmaterial covering said first and second surfaces, the surfaces of thelayer remote from said first and second surfaces being substantiallyflat.
 21. Apparatus according to claim 20 wherein said material has theproperty of partially diffusing the light interacting with said firstperiodic structure.
 22. Apparatus according to claim 3, wherein saidfirst periodic structure has two-dimensional extent in a referenceplane, and one of said first and second surfaces are parallel to saidreference plane.
 23. Apparatus according to claim 3, wherein said firstperiodic structure has two-dimensional extent in a reference plane andsaid first surfaces are parallel to said reference plane.
 24. Apparatusaccording to claim 3, wherein said first periodic structure hastwo-dimensional extent in a reference plane and said second surfaces areparallel to said reference plane.
 25. Apparatus according to claim 3,wherein said first periodic structure has two-dimensional extent in areference plane, said first surfaces are parallel to said referenceplane, the light between said light source and said read head has anincident path to, and a reflected path from, said first surfaces, andwherein the relative position of said light source and said read head issuch that one of said paths is at an angle normal to said referenceplane.
 26. Apparatus according to claim 3, wherein said first periodicstructure has two-dimensional extent in a reference plane, said firstsurfaces are at an angle oblique to said reference plane, the lightbetween said light source and said read head has an incident path to,and a reflective path from said first surfaces, and wherein the relativeposition of said light source and said read head is such that one ofsaid paths is normal to said reference plane.
 27. Apparatus according toclaim 3, wherein said first periodic structure has two-dimensionalextent in a reference plane, said first surfaces lie at an angle obliqueto said reference plane, said second surfaces lie at an angle oblique tosaid reference plane in a sense opposite to that of the first surfaces,and wherein the arrangement of said light source is such that lighttherefrom has a path substantially parallel to said second surfaces.